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Full text of "Text Book Of Mechanical Engineering"

Forms of Gates:                                 39

soundness of the casting being also in the hands of the pourer,
as he may keep the riser covered for a shorter or longer time.

The size of gates is determined by the fact that the metal
must neither flow too slowly so as to choke, nor too quickly so as
to break the mould. All the while the pouring is going on, the
moulder agitates the metal in the gates by means of an iron rod,
which he moves up and down until the metal has cooled so far
as to prevent him doing so any longer; in this manner homo-
geneous casting is more likely to result.

' A great deal of art lies in the
ramming of the mould, but as a
rule, the deeper portions of it should
be rammed most, as there will be
a greater head of metal on them.
The floor of the shop should be well
vented by a bed of coke and the
insertion of pipes, to take away the          ~ .                 -

gases  from all work  done  on  the                ^   "''"......'"

floor, and coke dust should be put

fn the joints of loam building; some cores too should have a

large amount of coke in their centre.

The venting of the mould is also a matter which, requires
a great deal of practical experience to enable it to be done
with success. Large cores, enclosed on three sides by metal
(' pockets,' moulders call them), should be particularly well
pierced, and green sand moulds should be much better vented
than loam or dry sand work, on account of the steam rising from
the damp sand and the compactness of the latter.

Cores require good support by means of plates or wires,
especially such as those in Figs. 25, 34, and 43, and all cores
that are not held down by the shape of the mould, should be so
fastened, for they are so much lighter than the molten metal,
that they would float out of position if left to themselves.

Cores should be dried in the stove for about twelve hours, and
should only be placed in the mould a short .time before pouring,
to prevent the absorption of moisture by them from the mould

Patterns are lifted out of the sand by screwing rods or
handles mto them (see Fig. l$d,jt>. 67), and raising slowly, at the